Laser diodes are used in optical transmission systems as light emitters. FIG. 1 illustrates a conventional laser diode behavior. Conventional laser diodes generate a characteristic optical power curve 110, three ranges can be distinguished. A pre-threshold portion A having a slight slope, in which there is no emission yet of coherent light. A “threshold area” B, which has a “knee” in the characteristic curve. Finally, a steeper portion of the characteristic curve includes a linear portion 111 having a constant slope, in which there is a linear correlation between the drive current and the optical power output.
As the temperature of the diode increases, its behavior changes. For example, the second laser power curve 120 depicts the optical performance of the same laser diode at a higher temperature. The characteristic curve appears to shift to the right (as indicated by the arrow). Additionally, the slope of the linear portion commonly becomes shallower. Also, the threshold current values increase.
Thus, when the operating point is set on a diode at one temperature it is valid for the diode only as long as it remains at the same temperature. However, in ordinary course of operation diode temperatures can fluctuate significantly. As shown in FIG. 1 this leads to significant changes in diode operating parameters (e.g., threshold current, optical power output, diode slope efficiency, as well as many other significant operating parameters). This makes calibration and operating points set at one temperature invalid at other temperatures. What is needed is method and apparatus for trimming and compensating for the effects of changing temperature. Moreover, it would be desirable to have a method and apparatus capable of utilizing previously obtained laser characterization data to facilitate the set up and compensation schemes of the present invention.